1. Field of the Invention
The present invention relates to a PAM (Pulse Amplitude Modulation) method and circuit for improving the performance of a D-class audio amplifier, and more particularly to a method and circuit capable of reducing the disadvantages caused by the dead time design in a D-class audio amplifier.
2. Description of the Prior Arts
Referring to FIG. 1, which is an illustrative view for showing a conventional D-class audio amplifier. Pulse Width Modulation (PWM) signal VPWM is inputted to NMOS 1 (N-Type Metal Oxide Semiconductor) and NMOS 2 located at the left side, and to NMOS 3 and NMOS 4 located at the right side of the D-class audio amplifier. Three inverters 5, 6 and 7 are arranged therebetween, as shown in the figure. The signals inputted to the gate of NMOS 1, NMOS 2, NMOS 3 and NMOS 4 are designated by TA+, TA−, TB+ and TB− respectively. The connecting point A between NMOS 1 and NMOS 2, and the connecting point B between NMOS 3 and NMOS 4 are output terminals, and a loudspeaker 8 (represented by an inductor) is connected between the connecting points A and B. Four diodes 9, 10, 11 and 12 are connected between a power source Vcc and a point N (low voltage point).
PWM signal VPWM is the square wave as shown at the bottom of FIG. 2, and the pulse width of the square wave represents the amplitude of the original analog signal Vsignal (the sine waves on the left side of FIG. 1 and at the top of FIG. 2). The analog signal Vsignal is cut by the triangular signal Vtriangle after passing through the comparator 13 at the left side of FIG. 1, and then is turned into the square wave VPWM.
To simplify the explanation, please refer to FIG. 3, the signal Vsignal is a constant voltage waveform instead of a sine wave, and the resulting ideal PWM waveforms inputted to NMOS 1 and NMOS 2 are indicated by TA+ and TA−, which are two waveforms inverted with each other. The waveforms of TB+ and TB− are functionally and morphologically identical to the waveforms TA+ and TA−, so further remarks will be omitted.
After this ideal PWM waveforms TA+ and TA− are inputted to NMOS 1 and NMOS 2, at the instant when TA+ and TA− change their status (point “a” in FIG. 3), NMOS 1 and NMOS 2 may be turned on simultaneously due to logic confusions. Thereby, a momentary high current will be produced to destroy NMOS 1 and NMOS 2. Similarly, NMOS 3 and NMOS 4 may also be destroyed by TB+ and TB−.
To prevent NMOS 1, NMOS 2, NMOS 3 and NMOS 4 from being destroyed, a circuit designer will delay the status change of the later waveform at the moment when TA+, TA−, TB+ and TB− change their statuses, thus forming a dead time, as shown by TA+(dt) and TA− (dt) in FIG. 3.
Although the dead time design can prevent NMOS 1, NMOS 2, NMOS 3 and NMOS 4 from being destroyed, diodes 9, 10, 11 and 12 will still be turned on. For example, if diode 9 is on (that means the inductor current is negative, iL<0), the waveform of VA (the voltage at point A in FIG. 1) will be changed in such a way as shown by the waveform at the bottom left of FIG. 3, the waveform used to decline at point a, but now it starts to decline after a period of time Δt, thus forming a “superfluous” area 31. If diode 10 is on (that means the inductor current is positive, iL>0), then the waveform of VA will change in such a way as shown by the waveform at the bottom right of FIG. 3, the waveform used to rise at point b, but now it starts to rise after a period of time Δt, thus forming a “loss” area 32.
Referring to FIG. 4, if the ideal output VA is a sine wave, then the real output will be distorted and unsmooth due to the influence of the dead time design, as shown by the dotted waveform in FIG. 4. This distorted real output will lead to an increase in total harmonic distortion (THD), thus spoiling the sound quality.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.